This invention relates to systems and apparatus for measurement of angular displacements relative to reference lines or datum planes. More particularly, the present invention relates to devices and systems for the measurement of horizontal angular displacements, vertical angular displacements or both. One aspect of the present invention is particularly useful for control of the collimating and focusing of light passing through multiple cells of a transparent/opaque coded disc. In yet another aspect of the invention accurate digital data is produced from a source of multiple bit digit groups coded in accordance with displacements from a reference line or datum plane with the resultant digital data production being relatively free of skew or alignment ambiguities associated with boundary transitions of those data bits. The invention is particularly useful for vertical angular measurement apparatus or inclinometers as will be described in detail for the preferred embodiment but its adaptability to other applications such as digital compass environments, linear displacement measurements and the like will be readily recognized by those having normal skill in the art.
Prior art devices and systems for measuring angular displacements have taken many forms. Directly readable mechanical scales as with plumb lines and arcuate members for inclinometers, low friction mounted circular discs for compasses or transits and the like have been satisfactory for many uses. Generally such measurements have been made relative to a zero or reference level position by manually leveling the instrument by observing bubble level vials or the like mounted on the instrument. The angular orientation or displacement is then read directly by moving some member of the instrument and observing scale alignments attached to the moving member relative to the fixed level member. Such predominantly mechanical devices have been employed in a variety of applications and industries such as in mining, road building, construction, military weaponry and so forth. For instance, in vertical angular control for military weaponry such as artillary pieces, the desired vertical angle of the barrel or tube is manually set by rotating a gear arrangement until the angle is shown by a mechanical digital readout. The barrel is subsequently raised or lowered until a bubble level vial indicates the level position has been reached.
Unfortunately, the accuracy of reading of such mechanical prior art devices is substantially limited. Thus, there have been some developments towards automating the digital data read-out associated with angular orientation of displacement measurement apparatus and converting the digital data to direct displays, data processing system input formats and the like. Optical systems associated with coded transparent/opaque discs or displacement reflecting members have been increasingly popular for improving the accuracy of data read. Further, the development of electronic circuitry for handling the data obtained from such optical systems has likewise increased the popularity of converting the resultant light correlated signals to readable displays or computer compatible input signals. One example of a wrap-around optical system associated with a circular code disc is shown in U.S. Pat. No. 3,244,895 by Anderegg. However, none of the known prior art devices satisfactorily concentrate the light sources from a light emitting device so as to produce a highly focused, narrow line of light for illuminating a coded transparent/opaque cell so as to maximize the transition of the light coupling or intervention by the cell and thus improve the accuracy of the optical read-out.
Various electronic systems have been developed for the purpose of providing digital data as an output in response to multiple bit input signals read from a code disc. It has been known that the use of the so-called excess -3 Gray code for such discs is particularly attractive since only one bit transition is encountered for each boundary between cells. Some systems for transposing the data read from such code discs are shown in U.S. Pat. Nos. 3,772,503 by Fowler and 3,833,901 by Fowler, both of which employ the disc output signals for controlling clock incremented pulse train generation to reflect the angular orientation or displacement being indicated by the disc at any given moment. A variation on such systems for accommodating angular displacement measurement or magnetic heading data by controlling the most significant digits of the disc to determine whether the conversion circuitry should be directly read or complemented is also shown to Fowler in U.S. Pat. No. 3,824,587. Other systems for electronically interpreting data such as Moire fringe pattern interpretation by incrementing up/down counters and further by parallel handling of the least and most significant digits so that the least significant digits can be preset and complemented is shown in U.S. Pat. No. 3,713,139 by Sanford et al. Yet another arrangement for parallel handling of least and most significant digits read from a code detector of sunlight angular measurement is shown in U.S. Pat. No. 3,728,715 by Shapiro.
Although not concerned with disc read-outs, U.S. Pat. No. 3,560,959 by Bergey shows a system for converting the output of an altitude encoder into a display by separate controls of the least and most significant bits. That is, systems such as that of Bergey use ripple counters for comparison against the encoder output for the more significant digits and logically interpret the least significant digits to determine both the status of a direct display of these least significant digits and also the direction of counting for the more significant digits.
Despite the popularity of transparent/opaque coded cells of discs and optical systems for use therewith and the application of electronic circuitry to convert the data acquired from such discs to an appropriate read-out, none of the prior art devices effectively overcome skewing or transition boundary misalignments of such discs or data sources or multiple bit groups having different data significance. As a practical matter, such misalignment or skew problems become increasingly critical as the width of the coded track narrows to accommodate the requirements for increasingly accurate angular measurement or displacement measurement. Prior efforts such as that disclosed in the U.S. Pat. No. 3,999,064 cross-referenced above are effective for somewhat reducing this misalignment problem by controlled sampling as near as possible to the center of the cells of the track. In addition, the increased number of bits that must be read from a given track and the physical constraints of the disc manufacture as well as the prospect of warpage or component misalignment during normal usage renders it difficult to obtain accurately read data from such tracks. The increasing of the diameter of the coded disc can partially overcome such limitations but again encounters practical limitations as to physical size for many applications. The use of sophisticated circuitry for cell center sampling is likewise undesirable both for manufacturing costs and for the reliability of usage in the field.